THE RESPONSE OF UNSATURATED SOILS TO ISOTROPIC STRESS

Soil compression is caused in agriculture by tillage implements, plant roots, treading by animals, and by wheels and tracks of vehicles. Increases in soil density resulting from compression usually reduce crop growth and yield. Compression and expansion of samples of five remoulded soils, each at several moisture contents, were investigated. Soil samples were subjected to isotropic stress of up to 3.5 MN m^?2 in a pressure cell. Volume changes were measured by the volume of pore fluid effused or infused through one of the sample surfaces. Particle packing densities, D, were well described by the equation where D₀ is the maximum limiting value of D, P is the applied isotropic stress, and B, C, K, L are adjustable parameters. One of the exponential terms in this equation describes deformation of soil crumbs and the other describes rearrangement of individual particles. Two sample sizes gave similar values for the equation parameters. A small increase in moisture content results in a large increase in soil compressibility. It is hypothesized that resistance to compression may be one of the principal influences in the mechanical restriction of root growth.